When creating or editing filters, on Tab 2 you will have the option to match on empty. Match on empty will include records where the field value matches or is blank.

IMPORTANT: Best practices determine that the “Match on Empty Field” should only be used for a secondary cleanse of the data. You will get the best results by cleansing the data first without selecting “Match on Empty Field,” and then selecting it for a later scan and cleanse of the data.